Suspension polymerization is used widely in industry as a process for producing rigid general-purpose thermoplastics such as vinyl chloride resin, styrene resin, acrylic resin and methacrylic resin. This is because polymerization heat can be easily controlled, and polymer particles can be recovered easily only by dehydration and drying.
However, when polymer particles are produced by suspension polymerization, the particle diameter usually shows broad distribution, and fine particle polymers having diameters deviated significantly from desired particle diameter are also simultaneously formed. As a result, there occur undesirable results such as extreme deterioration in filterability due to filter clogging caused by the fine powders in a dehydration step, entrainment of the fine powders in dehydration waste water, frequent troubles in the process due to clogging with the fine powders, deterioration in working atmosphere due to generation of powder dust, and an increase in danger of powder dust explosion.
The reason for generation of fine powder in suspension polymerization is that a monomer used in polymerization is water-soluble to some degree and is thus considerably emulsified upon initiation of suspension polymerization, or undergoes strong shear due to stirring so that monomer droplets are extremely over-dispersed. To solve these disadvantages, the type and amount of a dispersing agent added at the time of polymerization are selected for the former (see, for example, page 2, upper left column, line 12 to lower right column, line 2 in JP-A 49-88987), and for the latter, it has been examined to make the fluidized state uniform by improving a stirring blade. However, even if these techniques are simultaneously used, generation of fine particles is hardly completely suppressed in many cases, causing significant disadvantages to waste water disposal and the like by the producers.
Emulsion polymerization is used widely as a process for producing an impact-strength improver for rigid plastics such as vinyl chloride resin, styrene resin, acrylonitrile-styrene resin, polycarbonate resin and polyether resin. This is because it is considered that when graft copolymer resin after granulation and recovery is to be melted and blended with rigid plastics, the graft copolymer while maintaining the particle diameter thereof at the time of polymerization is then re-dispersed in the rigid plastics thereby exhibiting impact-resistant strength specifically.
To recover the desired graft copolymer from emulsion polymerization latex, a granulation procedure for recovering the latex by coagulation is usually necessary. This granulation procedure has a significant influence not only on the powder characteristics of the recovered particles (distribution of particle diameters, amount of fine powder, fluidity etc.) but also on productivity in subsequent processes, for example on dehydration properties and drying properties. Conventionally, in order to recover a particulate polymer from the latex produced by emulsion polymerization, the latex is usually converted into a solid-in-liquid dispersion by adding a coagulating agent to coagulate the polymer in the liquid phase and then subjecting the dispersion to a process such as heat treatment, and thereafter the dispersion is dehydrated and dried to obtain powdery particulate synthetic resin. In this case, however, the shape of the resulting powder obtained is indefinite to permit inclusion of a considerable amount of fine powder causing frequent troubles in the process and deteriorating working atmosphere attributable to generation of the powdery dust. Accordingly, various examinations for improvement, for example proposal of novel granulation methods such as a gaseous phase coagulation method (see, for example, page 1, lower left column, line 13 to page 2, upper left column, line 3 in JP-A 52-68285) and a mild coagulation method (see, for example, page 2, lower left column, line 8 to lower right column, line 2 in JP-A60-217224), have been proposed. In spite of such enormous efforts, the amount of water and electric power used in a coagulation step, washing step and drying step in the various granulation methods as described above is extremely enormous as compared with that of the suspension polymerization process. Thus they cannot be satisfactory in respect of energy saving and still there is demand for development for new granulation methods including design of resin powder.
On the other hand, a suspension polymer which is formed into articles by a molding machine and is used generally as a thermoplastic resin in the field of molding processing, is rarely processed alone, but usually quality improving agents such as impact-strength modifiers or processability improving aids are added thereto. These quality improving agents are usually produced by emulsion polymerization and recovered as powder as described above. Before molding processing, processors generally conduct a compounding procedure wherein a quality improving agent is added to and mixed with the suspension polymer, in order to obtain molded products of excellent physical properties. At present, however, this compounding procedure causes deterioration in working atmosphere due to generation of powdery dust and thus forcing the processors to reduce working efficiency.
As the technique related to the present invention, there are disclosed an electrostatic image-developing toner comprising a core particle formed by polymerization and a coating layer consisting of fine particles formed by emulsion polymerization on the surface of the core particle, which has excellent frictional charging properties and excellent moisture absorption, and a process for producing the same (see, for example, page 2, lower left column, line 13 to lower right column, line 9 in JP-A 57-45558). However, this method cannot solve the problem of fine powder attributable to suspension polymerization, because of the necessity of a procedure of once recovering the suspension polymer and subsequent coating with the emulsion polymer.
To produce a toner satisfying both toner sticking property and low-temperature fixation ability, there are disclosed an electrostatic image-forming toner comprising suspension polymer particles, 95° or more of the surface of which is coated with an emulsion polymer, as well as a method of producing the same (see, for example, column [0007] in JP-A 2000-112174). However, this method is usable in only the above purpose, and the average particle diameter of the suspension polymer described in the specification is as small as 2 to 10 μm, and this method is irrelevant to a method of solving the problem of fine powder in suspension polymerization in the present invention.
A method wherein as a polymer used in foamed products, a styrene polymer by suspension polymerization is coated with an emulsion polymer is disclosed (see, for example, column 1, line 59 to column 2, line 5 in U.S. Pat. No. 4,307,134; column 2, lines 2 to 22 in U.S. Pat. No. 4,333,969; column 2, lines 6 to 22 in U.S. Pat. No. 4,333,970; and column 2, lines 2 to 19 in U.S. Pat. No. 4,385,156). However, these methods are not those for solving the problem of fine powder generated in suspension polymerization, and the object of these inventions is completely different from that of the present invention.
A suspension polymer containing anti-blocking granules and having a glass transition temperature of lower than 50° C., wherein the anti-blocking granules have a anti-blocking coating consisting of an emulsion polymer having a glass transition temperature of 50° C. or more, as well as a process for producing the same is disclosed (see, for example, column [0010] in JP-A 6-179754). It is described therein that this process is a process which can be used only for preventing thermal adhesion of a suspension polymer having a low glass transition temperature, that is, high adhesion, and also that an excessive amount of coating with a emulsion polymer is removed by washing at the time of dehydration. Thus, this cited invention is different from the present invention solving the problem of fine powder in washing waste water.
Further, an emulsion-suspension polymerization method is disclosed as a method which comprises partially coagulating an emulsion polymer latex and adding an ethylene monomers thereto under stirring to convert an emulsion into a suspension, followed by suspension polymerization (see, for example, page 2, upper right column, lines 13 to 20, in JP-A 56-50907). In this method, composite particles wherein a suspension polymer used usually as thermoplastic resin and emulsion polymer particles as an impact-resistance improver have been integrated can be produced. When this technique is used, a coagulation (granulation) step essential for recovery of emulsion polymerization latex can be omitted and the resulting particles have an excellent spherical shape including a very small amount of fine powder. As granulated particles not necessitating high drying energy (low water content after dehydration) can be obtained, this method is more advantageous than the existing emulsion process in respect of energy consumption, so that the problems in the suspension polymerization and emulsion polymerization process can be significantly reduced. This method, however, is inferior in productivity, such as extreme increase in viscosity upon conversion of an emulsion to a suspension, formation of a polymerization scale and a significantly long polymerization time for completion of emulsion polymerization and subsequent continuous suspension polymerization.